A coherent optical receiver includes a local oscillator and mixes a received optical signal and a local oscillator light. A carrier wavelength of the received optical signal and a wavelength of the local oscillator light are approximately the same each other. A mixed light of the received optical signal and the local oscillator light is converted into an electric signal by a photo detector. The electric signal is amplified by an amplifier (for example, a Transimpedance Amplifier (TIA)) and then guided to a signal processing circuit. The signal processing circuit includes a demodulation circuit and so on, and recovers data from a received signal.
In order to improve a sensitivity of the photo detector in the coherent optical receiver, it is requested that an input optical power (or input optical intensity) of the photo detector be appropriately controlled. Thus, the coherent optical receiver is equipped with a variable optical attenuator (VOA) to adjust a power of the received optical signal. In addition, the coherent optical receiver is equipped with a monitor circuit that branches the received optical signal and monitors a power of the branched received optical signal. The variable optical attenuator is controlled based on the monitor result by the monitor circuit such that an optical signal with an appropriate power is guided to the photo detector.
When an optical signal of a specified wavelength (hereinafter referred to as a target wavelength) is selectively received from a wavelength division multiplexed (WDM) optical signal, the coherent optical receiver controls a wavelength of the oscillator light to be the target wavelength. By doing this, an electric signal that indicates an optical signal of the target wavelength is obtained by the photo detector.
Note that International Publication Pamphlet No. WO2012/066859 describes a configuration in which a variable optical attenuator is adjusted based on an output amplitude of an amplifier that amplifies an output signal of a photo detector. International Publication Pamphlet No. WO2015/004828 describes a configuration in which a variable optical attenuator is adjusted based on a gain control signal of an amplifier that amplifies an output signal of a photo detector. In addition, US Patent Publication No. 2013/0343751 and US Patent Publication No. 2013/0322876 describe related technologies.
When an optical signal of a target wavelength is selectively received from a WDM optical signal using a coherent optical receiver according to conventional technologies, it is difficult to precisely measure an input optical power of the target wavelength. For example, if a power of an input WDM optical signal is measured and the measured value is divided by a number of wavelength channels multiplexed in the WDM optical signal, an input optical power of one wavelength channel may be calculated. However, optical powers of respective wavelength channels multiplexed in the WDM optical signal are not equal to each other. Thus, the input optical power calculated using this method will include an error.
Note that if an optical filter that extracts an optical signal of a target wavelength is provided on the input side of the coherent optical receiver, an input power of the target wavelength may be precisely calculated. However, the configuration in which the optical filter is provided on the input side of the coherent optical receiver is not preferable for reducing a cost of communication equipment.